1. Field of the Invention
This invention relates to the field of communications, and particularly to communication systems using spread spectrum techniques and to over-the-air protocols for mobile telephones.
2. Description of Related Art
A mobile telephone system may generally comprise a set of xe2x80x9cuser stationsxe2x80x9d, typically mobile and the endpoints of a communication path, and a set of xe2x80x9cbase stationsxe2x80x9d, typically stationary and the intermediaries by which a communication path may be established or maintained. In a mobile telephone system, one important concern is the ability of mobile stations to communicate with base stations in a simple, flexible and rapid manner. The communication protocol between user stations and base stations should be rapid, so that user stations are not required to wait to establish a communication path. The protocol should be simple, so that user stations need not incorporate expensive equipment to implement it. The protocol should be flexible, so that user stations may establish communication paths in as many communication environments as reasonably possible.
Accordingly, it would be advantageous to provide a simple and flexible over-air protocol for use with a mobile telephone system. One class of systems in which this would be particularly advantageous is that of personal communication systems, particularly those with hand-held telephones in a microcell or other type of cellular communication system.
The invention provides in one aspect a simple and flexible over-air protocol for use with a mobile telephone system, such as a Personal Communication System (PCS) with hand-held telephones in a cellular communication system. A preferred, embodiment is adapted to xe2x80x9cpocket phonesxe2x80x9d, i.e., small hand-held telephones which may use a cellular communication technique, but the invention may be used with any cellular or mobile telephone system. The protocol defines a method in which user stations, such as cellular or mobile telephone handsets, communicate with one or more base stations to place and receive telephone calls.
The protocol provides air-channel agility between base stations and user stations, while providing a secure voice or data link and the ability to handoff calls between base stations while they are in progress.
In a preferred embodiment, each base station may have a set of xe2x80x9cair channelsxe2x80x9d which it polls, e.g. by transmitting to each one in sequence. The air channels supported by each base station are referred to as a xe2x80x9cpolling loopxe2x80x9d for a particular base station. A user station may receive information on an unoccupied air channel, receive the base station""a transmission, and transmit information to the base station. Each base station may therefore simultaneously maintain communication with as many user stations as there are air channels in its polling loop. The ability of a user station to communicate on any unoccupied air channel makes the protocol air-channel agile. Each base station continually transmits on each one of its air channels in a predetermined sequence. Each base station transmission may be followed by a first gap, a user station transmission (if some user station attempts to communicate), and a second gap, before the base station transmits on the next air channel. A base station transmission, first gap, user station transmission, and second gap are collectively called a xe2x80x9cminor framexe2x80x9d. A polling loop in which each air channel is polled is called a xe2x80x9cmajor framexe2x80x9d.
In a preferred embodiment, stability of user station and base station clocks may define the air channels, gaps, and minor frames. The user station may synchronize itself to the base station""a clock by detecting a minor frame and by adjusting its clock to be in synchrony with the base station when the first bit sequence of the minor frame is detected. The stability of the user station and base station clocks may then hold the user station and base station in synchronization, as long as the user station is periodically able to receive transmissions from the base station. Should reception in either direction be interrupted for too long, the base station and user station clocks may drift apart and the user station may need to reacquire the transmission from the base station.
Handoffs are preferably initiated from the user station which continually monitors available air channels from the same and competing base stations during dead time. A user station may handoff within the same polling loop to establish communication in a new minor frame, or may handoff in such a manner to establish communication in a new minor frame within a polling, loop of a different base station. In the latter case, a base station controller may assist in transferring the call from one base station to another.
The invention provides in yet another aspect for closed loop power control in the user stations by monitoring and adjusting the user station power at regular intervals, such as once in each major frame. The control of user station power serves to reduce intercell interference and prolong battery life in mobile handsets.
Variable data rates provided in another aspect of the present invention. A user station may increase its data rate by transmitting and/or receiving in multiple minor frames during a major frame, or may reduce its data rate by transmitting and/or receiving in fewer than every major frame.